This invention relates to a pump of the kind used for producing a smooth and continuous outflow of liquid at relatively high pressure.
This invention has particular application to a pump used for liquid chromatography.
In liquid chromatography the performance and data which can be obtained from the column and detector of the liquid chromatograph is dependent upon the characteristics of the flow of liquids supplied. A smooth and continuous flow of liquid at pressures up to 6000 psi is necessary to ensure repeatability and accuracy of chromatographic data.
When the liquid supplied to the column and detector is formed as a composition of solvents, performance is also dependent upon accuracy and smoothness of the liquid composition.
The pressurization of the liquid is typically accomplished by piston type pumps. Single piston pumps and multiple piston pumps have been used in the prior art.
Single piston pumps are inherently pulsating type flow devices. Single piston pumps therefore present problems in achieving both of the above requirements.
The single piston pumps have required auxiliary devices to smooth out the pulsating flow. Hydraulic capacitor (such as bourdon tubes) type devices have been used with single piston pumps to try to smooth out the flow, but these type devices do not produce the essentially pulseless flow desired.
Because the single piston pump has no flow at the beginning of the pump cycle (during its intake stroke), single piston pumps have usually been constructed to provide a very fast intake stroke. This causes poor accuracy of composition when the liquid composition being pressurized is formed by proportioning valves at the inlet of the pump. Using a very short intake stroke for the single piston pump makes timing and actuation of the proportioning valves unnecessarily critical. To get good accuracy of composition forming it is better to have a suction stroke which is of sufficient duration so that any inaccuracies in the timing of the solvent proportioning valves do not become appreciable. This long intake stroke which is desired for accuracy of composition forming conflicts with the short intake stroke which is desired for minimizing pulsations in the outflow of a single piston pump.
The prior art has also used a two piston pump with the pistons flow connected in parallel to try to avoid the pressure pulsations in the outflow. In one prior art pump construction of this kind, the two pistons have been driven by one cam with the two pistons located on opposite sides of the drive came so as to be driven 180.degree. out of phase with one another. These dual piston pumps provide almost pulseless flow but are more complex than single piston pumps. They require an inlet valve and an outlet valve for each piston.
This parallel piston pump arrangement of the prior art can present problems in obtaining pulseless flow because of compressibility of the liquid being pumped. In looking at the outflow from the pump there is a decrease in flow at the start of the expulsion stroke of each piston, and the amount of the decrease is largely a function of the compressibility of the liquid and the pressure.
One prior art technique which was developed to compensate for this decrease in flow at the start of the expulsion stroke was to speed up the pump motor in response to the dip. Mechanical inertia, friction, and hydraulic loading are factors which limit the achievable motor acceleration. Increasing the speed of a stepper motor rapidly enough under these conditions at the beginning of the displacement stroke can be a problem.
This parallel piston pump arrangement can also present problems in obtaining accuracy and smoothness of composition when the liquid composition is formed by proportioning valves at the inlet of the pump. Because of the parallel flow connection, a change in inlet composition will not be accomplished smoothly at the pump outlet. After one piston delivers the new composition, the second piston will also deliver the new composition but this will be preceded by the old composition contained in the flow passage between the second piston and the junction with the flow passage from the first piston.